1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a transflective liquid crystal display and method of fabricating the same.
2. Discussion of the Related Art
In general, both transmission and reflection type liquid crystal displays (LCD) are used. Transmission type LCDs use internal light sources while reflection type LCDs use external light sources. The transmission type LCD has a liquid crystal display panel, which does not emit light itself, and has a backlight as a light-illuminating section.
The backlight is disposed at the rear or one side of the panel. The amount of the light from the backlight that passes through the liquid crystal panel is controlled by the liquid crystal panel in order to implement an image display. In other words, the light from the backlight varies and displays images according to the arrangement of the liquid crystal molecules. However, the backlight of the transmission type LCD consumes 50% or more of the total power consumed by the LCD device. Providing a backlight therefore increases power consumption.
In order to overcome the above problem, a reflection type LCD has been selected for portable information apparatuses that are often used outdoors or carried with users. Such a reflection type LCD is provided with a reflector formed on one of a pair of substrates. Thus, ambient light is reflected from the surface of the reflector. The reflection type LCD using the reflection of ambient light is disadvantageous in that a visibility of the display is extremely poor when surrounding environment is dark.
In order to overcome the above problems, a construction which realizes both a transmissive mode display and a reflective mode display in one liquid crystal display device has been proposed. This is so called a transflective liquid crystal display device. The transflective liquid crystal display (LCD) device alternatively acts as a transmissive LCD device and a reflective LCD device. Due to the fact that a transflective LCD device can make use of both internal and external light sources, it can be operated in bright ambient light as well as has a low power consumption.
FIG. 1 shows a related art transflective liquid crystal display (LCD) device, and FIG. 2 is a plan view showing the related art transflective LCD device of FIG. 1. The transflective LCD device 10 includes upper and lower substrates 80 and 60 with an interposed liquid crystal layer 95. The upper and lower substrates 80 and 60 are sometimes respectively referred to as a color filter substrate and an array substrate.
On the surface facing into the lower substrate 60, the upper substrate 80 includes, in series, a color filter layer 90 and a common electrode 86. The color filter layer 90 includes a plurality of sub-color filters 82 and a black matrix 84. The sub-color filters 82 includes a matrix array of red, green, and blue color filters and the black matrix 84 is disposed among the sub-color filters 82, such that each sub-color filter 82 is divided by the black matrix 86. The common electrode 86 is over the sub-color filters 82 and the black matrix 84.
On the surface facing into the upper substrate 80, the lower substrate 60 includes an array of thin film transistors (designated as TFT “T” in FIG. 1) that act as switching devices. The array of thin film transistors is formed to correspond to the matrix of sub-color filters. A plurality of gate and data lines 61 and 62 are positioned and crossed over each other. A TFT is located near at each crossing portion of the gate and data lines 61 and 62. The lower substrate 60 also includes a plurality of pixel regions P that are defined by the crossing of the gate and data lines 61 and 62, as shown in FIG. 1. Transparent and reflective electrodes 64 and 66 are disposed in the pixel regions P. Each pixel region P is divided into a transmissive portion B and reflective portion D. The transmissive portion B is located in the middle of the reflective portion D. The reflective electrode 66 is disposed corresponding to the reflective potion D, and the transparent electrode 64 is disposed in the pixel region P with covering the transmissive portion B.
The transparent electrode 64 is usually formed of a transparent conductive material having good light transmissivity, such as indium tin oxide (ITO). The reflective electrode 66 is formed of a metallic material having a good reflectivity, such as aluminum (Al) or aluminum alloy.
FIG. 3 is a cross sectional view taken along a line III—III of FIG. 1 and illustrates a transflective LCD device according to a related art.
As shown, the first (lower) substrate 60 is spaced apart from and faces to the second (upper) substrate 80. The liquid crystal layer 95 is interposed between the first and second substrates 60 and 80. As described with reference to FIGS. 1 and 2, the plurality of pixels P, which are defined by the gate and data lines 61 and 62 both perpendicularly crossing to each other, are formed in the first and second substrates 60 and 80.
On a rear surface of the second substrate 80, formed are sub-color filters 82a and 82b each having one of the red, green and blue colors. The black matrix 84 is disposed between the sub-color filters 82a and 82b. The common electrode 86 is disposed on the rear surface of the sub-color filters 82a and 82b and the black matrix 84.
Each of the pixel regions P is divided into the reflective portion D and the transmissive portion B. In general, the reflective electrode 64 is formed within the reflective portion D, and the transparent electrode 66 is formed to correspond to the transmissive portion B. The reflective electrode 64 is usually formed over or under the transparent electrode 66. In FIG. 3, the reflective electrode 64 has a light-transmitting hole H that corresponds to the transmissive portion B, and the reflective electrode 64 is disposed under the transparent electrode 66.
In the transflective LCD device of FIG. 3, it is important that a color difference should not appear in between the transmissive portion B and the reflective portion D. Further, the transmissive and reflective portions B and D should have the same optical efficiency. However, an incident light traveling the reflective potion D passes the color filter twice because the incident light passing through the color filter reflects on the reflective electrode 64 and then proceeds towards the color filter again. Therefore, if the liquid crystal layer 95 has a cell gap “d” between the two substrates, the incident light makes a trip along a distance “2d” while a light passing through the transmissive portion B only has a “d”-distance journey. The phase retardation of the light can be represented by 2dΔn (2d.delta.n) when the light passes the liquid crystal layer 95 in the reflective potion D. On the contrary, the phase retardation of the light can be represented by dΔn (d.delta.n) when the light passes the liquid crystal layer 95 in the transmissive potion B. As a result, the reflective and transmissive portions D and B have difference phase retardation values such that it is impossible that the same color purity appears in both the reflective and transmissive portions D and B.
To solve the above problems, it is suggested that the transmissive portion B has a different cell gap substantially twice as much as the distance “d”. Namely, the transflective LCD device is designed to have a first cell gap “d” in the reflective portion D and a second cell gape “2d” in the transmissive portion B. These difference cell gaps will now be explained with reference to FIG. 4.
FIG. 4 is a cross sectional view taken along a line III—III of FIG. 1 and illustrates the transflective LCD device according to another related art.
As shown, the first (lower) substrate 60 is spaced apart from and faces to the second (upper) substrate 80. The liquid crystal layer 95 is interposed between the first and second substrates 60 and 80. As described with reference to FIGS. 1 and 2, the plurality of pixels P, which are defined by the gate and data lines 61 and 62 both perpendicularly crossing to each other, are formed in the first and second substrates 60 and 80.
On a rear surface of the second substrate 80, formed are sub-color filters 82a and 82b each having one of the red, green and blue colors. Additionally, the black matrix 84 is disposed on the second substrate 80 between the sub-color filters 82a and 82b. The common electrode 86, which is transparent, is disposed on the rear surface of the sub-color filters 82a and 82b and the black matrix 84.
Like the transflective LCD device shown in FIG. 3, each of the pixel regions P is divided into the reflective portion D and the transmissive portion B. The reflective electrode 64 is formed within the reflective portion D, and the transparent electrode 66 is formed in the pixel region P with corresponding to both the transmissive portion B and the reflective potion D. The reflective electrode 64 is usually formed over or under the transparent electrode 66. In FIG. 4, the reflective electrode 64 has a light-transmitting hole H that corresponds to the transmissive portion B, and the reflective electrode 64 is disposed under the transparent electrode 66. Therefore, an area where the reflective electrode 64 is disposed is defined as a reflective portion D.
Unlike the transflective LCD device of FIG. 3, the transflective LCD device of FIG. 4 has a thick insulator 63 that has openings 61 in the transmissive portions B. Namely, each opening 61 is formed to correspond to the transmissive potion B in the insulator 63 such that the reflective and transmissive portions D and B have different cell gaps “d” and “2d”. The liquid crystal layer 95 has a first cell gap “d” in the reflective potion D and a second cell gap “2d” in the transmissive portion B. If the insulator 63 is as thick as the first cell gap “d”, the second cell gap “2d” will be a double than the first cell gap “d”. Namely, since the thickness ratio of the transmissive portion B to the reflective portion D is 2d to d, the phase retardation becomes the same of 2dΔn (2d.delta.n). Furthermore, although not shown in FIG. 4, the reflective electrode 64 can have an uneven surface (with prominences and depressions) to increase the reflectivity thereof.
However, the transflective LCD device illustrated with reference to FIG. 4 may have some light leakage around an interface between the transmissive potion B and the reflective portion D. Such light leakage will be explained with reference to FIGS. 5 and 6.
FIG. 5 is a plan view illustrating one sub-pixel of an array substrate for use in a transflective LCD device according to a related art, and FIG. 6 is an enlarged cross section view illustrating a portion K of FIG. 5.
As shown, a gate line 61 is disposed in a horizontal direction over a substrate 60, and a data line 62 is disposed in a longitudinal direction perpendicularly crossing the gate line 61. The gate and data lines 61 and 62 cross each other to define a pixel region P. A thin film transistor T, which includes a gate electrode 70, an active layer 72 and source and drain electrodes 74 and 76, is disposed near a crossing of the gate and data lines 61 and 62. Like the previous description, the pixel region P is divided into a reflective portion D and a transmissive portion B. A reflective electrode 66 is formed in the reflective portion D and has an opening H in the middle thereof. A transparent electrode 64 is formed to correspond to the pixel region P, especially to cover the opening H that corresponds in size to the transmissive portion B. The opening has a width W and a length L.
Still referring to FIGS. 5 and 6, an insulator 63 has an opening that corresponds to the opening of the reflective electrode 66 and forms a step “d” to make different cell gaps, as illustrated with reference to FIG. 4. At this point, the insulator 63, however, has a slope 63a. Due to the step “d” and the slope 63a, a disclination may occur in an area F that is between the reflective portion D and the transmissive portion B. As shown in FIG. 6, the disclination occurs both in a first disclination area FI corresponding to the slope 63a and in a second disclination area F2 next to the slope 63a. The disclination area extends from the slope area F1 to the neighboring area F2.
In FIG. 6, if the slope 63a has a height d of about 2 micrometers and forms an angle θ of 50 degrees with the base, the bottom side of the triangular cross section will have a distance of about 1.7 micrometers (calculated by d/tan θ). Furthermore, the second disclination area usually has a distance of about 1.5 micrometers.
Accordingly, the disclination area F has a distance of about 3.5 micrometers (1.7+1.5). And thus, the dimensions (A) of the disclination area F will be represented by the following equation: A=2(L+W)×3.2 μm2 (where L is the length of the opening and W is the width of the opening, as shown in FIG. 5).
Thus, the larger the opening, the bigger the disclination. Usually, the disclination area caused by the step and slope reduces the aperture ratio by about 10% in the transflective LCD device shown in FIGS. 4–6. This means that the brightness and the contrast ratio of the transflective LCD device are lowered correspondingly.
Moreover in the related art transflective LCD device, since the opening and the transmissive portion are relatively small and are located in the middle of the pixel region P, it is difficult to uniformly perform the rubbing process when inducing the initial alignment direction of the liquid crystals. This prevents uniform and stable transmittance from being attained in the transmissive portion.